Turbo coding and turbo decoding were discovered in 1993 (see Non-Patent Document 1). They are attracting attention as error correction coding that approaches a Shannon limit, which is a theoretical limit in communication.
An apparatus in which turbo coding is implemented (hereinafter, referred to as “turbo coding apparatus”) includes two recursive systematic convolutional (RSC) coding units which have two common wire connections. The turbo coding apparatus inputs an input bit stream to the two RSC coding units. In this case, the turbo coding apparatus interleaves the bit stream through an interleaver and inputs it to one of the RSC coding units. Through this process, the turbo coding apparatus generates the bit stream which is convoluted under two independent constraint conditions.
An apparatus in which turbo decoding is implemented (hereinafter, referred to as “turbo decoding apparatus”) includes two maximum a posteriori probability (MAP) estimation units. The two MAP estimation units decode the bit stream which is encoded by the turbo coding apparatus. The MAP estimation units detect the input bit stream with a high degree of accuracy by exchanging reliabilities obtained by respective decoding processes with each other.
As a scheme that focuses on two independent constraint conditions, a turbo equalization scheme has also been suggested. In the turbo equalization scheme, a coding process as a premise is executed by two coding units as follows: One performs convolutional coding (external coding) of a bit stream having the purpose of error correction. The other performs convolutional coding (internal coding) by an impulse response of a propagation path. The two independent coding units are connected in series.
An apparatus that receives the signal coded by the above-described coding process (hereinafter, referred to as “turbo equalization apparatus”) includes an equalization unit which compensates distortion by the radio propagation path and a decoding unit which performs decoding. The equalization unit and the decoding unit use reliabilities obtained by their processes as prior information for their detection process. By repeating such a process, the turbo equalization apparatus gradually improves the accuracy of signal detection.
As an example of the turbo equalization scheme, a frequency-domain soft canceller followed by minimum mean square error (SC/MMSE) turbo equalization scheme will be described. FIGS. 16 and 17 illustrate a transmitting apparatus and a receiving apparatus for radio communication, particularly, mobile communication.
First, the transmitting apparatus will be described. In FIG. 16, the transmitting apparatus includes a coding unit P01, an interleaver P02, a modulation unit P03, a cyclic prefix (CP) insertion unit P04, a pilot generation unit P05, a pilot multiplexing unit P06, a radio unit P07, and a transmitting antenna P08.
The coding unit P01 executes error-correction coding for a bit stream to be transmitted (hereinafter, referred to as “transmission bit stream”). The interleaver P02 interleaves a bit sequence of the transmission bit stream. The modulation unit P03 performs modulation such as quadrature phase shift keying (QPSK) on the transmission bit stream whose sequence is interleaved. The CP insertion unit P04 inserts a CP into the transmission bit stream. The CP is aimed at removing interference of a delay wave caused by a multipath. In this case, the CP insertion unit P04 adds the CP in units of fast Fourier transform (FFT) blocks in the receiving apparatus depending on a maximum delay time of a propagation path.
The pilot generation unit P05 generates a known pilot signal for propagation path estimation. The pilot signal multiplexing unit P06 multiplexes a signal of the transmission bit stream into which the CP is inserted (hereinafter, referred to as “data signal”) and the pilot signal generated by the pilot generation unit P05. The radio unit P07 up-converts the multiplexed signal into a radio frequency. The transmitting antenna P08 transmits the up-converted signal.
Next, the receiving apparatus will be described. In FIG. 17, the receiving apparatus includes a receiving antenna P11, a radio unit P12, a pilot separation unit P13, a propagation path characteristic/variance estimation unit P14, a CP removal unit P15, an FFT unit P16, a soft cancellation unit P17, an equalization unit P18, a demodulation unit P19, a de-interleaver P20, a decoding unit P21, an interleaver P22, a soft replica generation unit P23, and a propagation path characteristic multiplying unit P24.
The receiving antenna P11 receives a signal transmitted from the transmitting antenna P08 of the transmitting apparatus described above. The radio unit P12 down-converts the received signal from the radio frequency to a baseband signal. The pilot separation unit P13 separates the down-converted signal into the pilot signal and a data signal. The propagation path characteristic/variance estimation unit P14 estimates a frequency response of the propagation path (hereinafter, referred to as “propagation path characteristic”) using the pilot signal. Further, the propagation path characteristic/variance estimation unit P14 estimates variance of thermal noise (hereinafter, referred to simply as “noise”) of the receiving apparatus. The propagation path characteristic and the variance of thermal noise estimated are input to the equalization unit P18. Further, the estimated propagation path characteristic is input to the propagation path characteristic multiplying unit P24.
The CP removal unit P15 removes the CP from the separated data signal. The data signal maintains a periodic function characteristic of the signal in units of FFT blocks. The FFT unit P16 converts the data signal into the frequency signal. The frequency signal is input to the soft cancellation unit P17.
The processing processed hereafter is signal processing in the turbo equalization scheme. The turbo equalization scheme involves repeating processes in the soft cancellation unit P17, the equalization unit P18, the demodulation unit P19, the de-interleaver P20, the decoding unit P21, the interleaver P22, the soft replica generation unit P23, and the propagation path characteristic multiplying unit P24 an arbitrary number of times.
First, the soft cancellation unit P17 cancels a reception signal replica (a signal of an interference component in the propagation path) having an amplitude that is in proportion to reliability obtained by the propagation path characteristic multiplying unit P24, from the data signal. In a first process, since the soft replica is not generated by the soft replica generation unit P23, the reception signal replica is not generated. For this reason, the soft cancellation unit P17 does not perform a cancellation process. The equalization unit P18 receives the propagation path characteristic output by the propagation path characteristic/variance estimation unit P14, the soft replica obtained by the soft replica generation unit P23, and the data signal cancelled by the soft cancellation unit P17. The equalization unit P18 executes a process of compensating distortion of the signal in the propagation path (equalization process) using the propagation path characteristic, the soft replica, and the soft-canceled data signal. The demodulation unit P19 computes a log likelihood ratio (LLR) of each receiving data signal through a demodulation process.
The de-interleaver P20 restores the bit stream of the data signal to the original sequence. The decoding unit P21 executes an error correction process for the LLR of each data signal restored to the original sequence and computes the LLR of each data signal with improved reliability. The interleaver P22 re-interleaves the LLR output by the decoding unit P21 again. The interleaver P22 performs interleaving in the same pattern as the interleaver P02 of the transmitting apparatus. The soft replica generation unit P23 generates the replica having an amplitude which is in proportion to reliability. For example, if the QPSK, which is the modulation scheme in the modulation unit P03 of the transmitting apparatus, is used as a modulation scheme, and the LLR of a first bit constituting a QPSK signal is 11 and the LLR of a second bit is 12, the soft replica ssoft is expressed by Formula 1.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      s          soft                =                                            1                              2                                      ⁢                          tanh              ⁡                              (                                                      l                    1                                    2                                )                                              +                      j            ⁢                          1                              2                                      ⁢                          tanh              ⁡                              (                                                      l                    2                                    2                                )                                                                        (                  FORMULA          ⁢                                          ⁢          1                )            
When the soft replica is obtained by Formula 1, the soft replica generation unit P23 inputs the soft replica to the equalization unit P18. Further, the soft replica is converted into the frequency signal by an FFT unit of the soft replica generation unit P23 and input to the propagation path characteristic multiplying unit P24. The propagation path characteristic multiplying unit P24 generates the reception signal replica by multiplying the frequency signal of the soft replica by the propagation path characteristic estimated by the propagation path characteristic/variance estimation unit P14. The generated reception signal replica is input to the soft cancellation unit P17 as described above. The receiving apparatus repeats the above-described process an arbitrary number of times and finally determines the LLR obtained by the decoding unit P21 to perform signal detection, thereby obtaining the decoded bit stream (hereinafter, referred to as “decoded bit stream”).
[Non-Patent Document 1] C. Berrou, A. Glavieux, and P. Thitimajshima, “Near shannon limit error correcting coding and decoding: Turbo-codes (1),” in Proceedings of IEEE International Conference on Communications '93, (Geneva, Switzerland), pp. 1064-1070, May 1993.